The present invention relates to improvements in compact fluorescent lamps, and, more particularly, relates to improvements of the fluorescent lamps of this type in which darkening of the surfaces of outer bulbs and/or inner tube means is prevented, thereby to improve lumen output maintenance of the lamps.
Recently, fluorescent lamps with an Edison type base which can be directly inserted in Edison-base-sockets, i.e., so-called compact fluorescent lamps, have been extensively developed. Of these compact fluorescent lamps, there has been known, as disclosed for example in U.S. Pat. No. 4,199,708, a compact fluorescent lamp which comprises an outer bulb filled with a rare gas and mercury, and U-shaped inner tube means enclosed in the outer bulb, the inner tube means having their respective one ends formed with openings opened to the space inside the outer bulb and their respective other ends provided with electrodes. The above-mentioned fluorescent lamp is advantageous in that a high lumen output can be obtained without enlarging its diameter and without diminishing efficiency because the vapor pressure of mercury depends on the temperature of the outer bulb. The initial characteristics of the fluorescent lamp are satisfactory. Such a fluorescent lamp is, however, disadvantageous in that when the fluorescent lamp is used for a long time, the inner surface of the outer bulb and the outer surfaces of the inner tube means become darkened, so that the lumen output is decreased and so-called lumen output maintenance deteriorates.
On the other hand, U.S. Pat. No. 4,199,708 describes that light dispersing material, such as finely dispersed Ca.sub.2 P.sub.2 O.sub.7 or TiO.sub.2, is provided on the inner surface of the outer bulb to scatter visible rays radiated from the inner tube means. Further, Japanese Patent Unexamined Publication No. 57253/1983 describes that selective light transmitting material, such as CoO, Al.sub.2 O.sub.3 or TiO.sub.2, is provided on the inner surface of the outer bulb or the inner surfaces of the inner tube means to transmit specific wave lengths of rays. Furthermore, Japanese Patent Unexamined Publication No. 180953/1984 describes that the outer surfaces of the inner tube means are partly coated with 6 .mu.m thick Al.sub.2 O.sub.3 with particle diameter of about 0.02 .mu.m to prevent darkening of the interior tubes. Each of these conventional examples is advantageous in improving the non-darkening characteristic of the inner surface of the outer bulb or the outer surfaces of the inner tube means and in improving lumen output maintenance. However, each of these conventional examples is disadvantageous in that initial lumen output becomes lowered because the material provided on the inner surface of the outer bulb or on the outer surfaces of the inner tube means scatters or absorbs visible rays.